Polycaprolactones are polymers which are of a certain industrial interest in various fields, due in particular to their biocompatibility, their physicochemical properties and their good thermal stability up to temperatures of at least 200-250° C.
Conventionally, the preparation of homopolymers by ring opening involves ionic polymerization mechanisms or coordination/insertion polymerization mechanisms. These polymerizations are mainly initiated by metal complexes, such as alkyl- or arylalkali metals (in the case of anionic polymerization) or metal alkoxides (in the case of coordination/insertion polymerization). In the latter, mention may be made of aluminum, tin, zinc, iron, scandium, titanium or yttrium derivatives. The metal derivatives may possibly assist in the attack of the alkoxide on the monomer by coordination of the reactive functional group of the ring (ester or amide functional group respectively in the case of lactones or lactams). Penczek et al. describe, in Macromol. SymP, 128, 241-254 (1998), the preparation of polyesters by ring opening in the presence of aluminum alkoxides. In 2000, Hawker et al. provided for the opening of lactams by tin, titanium and magnesium complexes (Macromol. SymP, 157, 71-76 (2000)). In 2002, Jérôme et al. described the preparation of polylactones and polylactides by ring opening using dialkylaluminum alkoxides (Macromol. SymP, 177, 43-59 (2002)). More recently, patents JP2005042059 and JP2005042058 claimed the use of titanium derivatives (titanium tetraisopropoxide) as metal catalysts for opening the ε-caprolactone ring.
However, the presence of metal compounds employed in these polymerization processes can have a harmful effect on the stability and/or the performance of the polymers synthesized as a result of potential interactions with the polymer matrix or with other components participating in the formulation in the application. Furthermore, it is well known that metal salts catalyze the decomposition of polymer matrices, such as polycarbonate, during the use thereof or are undesirable when these polymers are used in biomedical applications.
It is therefore necessary to carry out a stage of purification of the final reaction medium in order to remove residual metal traces. This postpolymerization treatment stage is particularly problematic and can prove to be expensive for an effectiveness which is sometimes debatable.
Alternative processes which do not resort to metal catalysts have consequently been provided. These processes employ acids which act as catalyst by activating the reactive functional group of the monomer (ester or amide functional group respectively in the case of lactones or lactams). These cationic polymerization mechanisms then make it possible to dispense with the use of organometallic complexes in the reaction medium.
In particular, it has been suggested, by Endo et al., Macromol., 2000, 33, 4316-4320 and Jérôme et al., Macromol., 2002, 35, 1190-1195, to polymerize ε-caprolactone in the presence of ethereal hydrochloric acid (HCl.Et2O) and of n-butanol in dichloromethane at 25° C. The first authors use a monomer concentration of 1 mol.l−1 and up to 5 equivalents of acid with respect to the alcohol and obtain, after 24 h, polymers with maximum weights Mn of 10 300 g/mol (measured by gel permeation chromatography or GPC), with a polydispersity index of 1.15. The second authors use a monomer concentration of 4 mol.l−1 and 3 equivalents of acid with respect to the alcohol, and the polymers obtained after 29 h exhibit maximum weights Mn of 11 000 g/mol (i.e., approximately 20 000 with polystyrene calibration) and a polydispersity index of 1.25.
In the literature, only polycaprolactones having molecular weights Mn of less than 15 000 g/mol have been synthesized according to the above process. In addition, the use of this process requires very long reaction times, which negatively affect the economics of these processes, and the use of corrosive acid, which may detrimentally affect the equipment used.
Other processes for the cationic polymerization of ε-caprolactone have been provided which involve a sulfonic acid as catalyst instead of hydrochloric acid.
Thus, Jones et al., Macromol., 2004, 37, 9709-9714, have published the polymerization of ε-caprolactone in the presence of para-toluenesulfonic acid and of benzyl alcohol in toluene at 52° C. in order to result, after 5 h 30, in polymers with weights Mn of less than 9500 g/mol with a polydispersity index of 1.61. They have also used n-propylsulfonic acids supported on silica under the same conditions but the polymers obtained exhibited weights of less than 6500 g/mol for polymerization times of greater than 27 h.
Neither do these processes make it possible to prepare polycaprolactones having a possibly high molecular weight Mn with a low polydispersity index. This is because the reaction is usually difficult to control, it being possible in particular for undesirable transesterification reactions to occur, so that it is not always easy to obtain polymers having the expected homogeneity in chain length. In addition, these processes exhibit the disadvantage of having to be carried out under hot conditions, which can be harmful to their economics.
Another process for the homopolymerization of caprolactone has been described by Maigorzata Basko et al. in Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 7071-7081 (2006). It consists in reacting ε-caprolactone in the presence of isopropyl alcohol and of trifluoromethanesulfonic (triflic) acid in dichloromethane at 35° C. A polycaprolactone having a molecular weight Mn of 3100 g/mol and a polydispersity index of 1.05 can thus be obtained. While this process makes it possible to limit the transesterification reactions, it exhibits, however, the disadvantage of having excessively slow kinetics, in particular for high molecular weights Mn.